Re: Proposal: Remove Semigroup and Monoid instances for Data.Map, Data.IntMap, Data.HashMap

On 2018-02-14 04:08 AM, Evan Laforge wrote:

On Wed, Feb 14, 2018 at 12:49 AM, Akio Takano wrote:

...

I'm against this proposal because I think the improvement is, if not negative, too small to justify breaking existing code.

I agree for the same reasons.

I also find that unions is the most common. In my code, I have many uses of (<>) with the union meaning... it's hard to grep for, but I guess in the 40-80 range. Meanwhile, I have a Util.Map.mappend that also mappends the values, and it's used in 2 places (out of around 140k lines). It wouldn't be so bad to have to convert them all to Map.union, but doesn't seem worth the bother either.

I understand it's hard to grep, but can you guess how many of those 40-80 uses of (<>) with the union meaning actually depend on the left-biased semantics? I've looked at my code, and it turns out that wherever this operation appears, the two maps are guaranteed to have distinct keys. There is actually one use I'm not sure about, I'll have to make sure there's no bug hiding there. That is one reason I'm supporting the proposal, by the way: the existing behaviour that silently drops values is much more likely to cause bugs.